email spam blocking for isp hosting apache

Avg Bid (USD)
Project Budget (USD)
$30 - $250

Project Description:
I have a hosting www server with several hundreds of custommers, php, postfix. Often some of users have an evil script which generate a spam. I need a solution for avoiding or blocking it without disable users to send legitimate emails from server. Give me overview of your solution and your expertise.

Skills required:
Apache, PHP, System Admin
About the employer:
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